Afocal optical systems are used in a variety of applications. An afocal foreoptical system may be derived from the first two mirrors of a focal three-mirror anastigmat optical system, such as a focal three-mirror anastigmat telescope. An example of a derived all-reflective afocal optical system is described in U.S. Patent Publication No. 2010/0202073 published on Aug. 12, 2010, which is herein incorporated by reference in its entirety.